The proposed case-control study is designed to investigate dietary factors which may inhibit cancer of the colon and rectum. These include vitamin C, vitamin E, vitamin A (retinol), beta carotene, selenium and protease inhibitors. Dietary intake of fats and fibers will additionally be assessed. The cases will be about 350 newly diagnosed colorectal cancer patients at three hospitals. A series of 350 patients with non-neoplastic disease from these hospitals will serve as controls. An additional population-based control group will also be obtained. Questionnaire information will be obtained on non-dietary risk factors for colon cancer and on sociodemographic variables. Dietary intake will be assessed by an extensive dietary interview which will be directed toward recall of intake 2-3 years ago (preceding symptomatic disease). Nutrient intakes will be calculated from an extensive and up-to-date food composition database for most nutrients. In order to confirm the validity of dietary intake information and to ascertain more physiologically-relevant parameters, bilogical assays will be conducted for about 100 cases and 100 controls. Fecal samples will be obtained and assayed for levels of protease inhibitors. Hair, which provides a temporally-integrative measure of various trace metals, will be assayed for levels of selenium. Since several of the dietary factors in question have been shown to have joint potentiating or compensating effects, interactions among them will be investigated in addition to their separate inhibitory effects. Finding a reduction in risk associated with some or all of these dietary factors would point to dietary modification as a potential preventive measure for cancer of the colon and rectum.